1. Field of the Invention
The present invention relates to a precursor solution for forming a thin film of ferroelectric substance composed of a composite oxide having a perovskite structure containing a lead such as PZT(Pb(Zr, Ti)O3) or PLZT(PbO-La2O3-ZrO2-TiO2), and to a production process for producing such a precursor solution.
2. Prior Art
A thin film of perovskite group such as PZT exhibits a high permittivity or ferroelectricity and, particularly because of its superior P-E hysteresis characteristic, the thin film is used as an essential element of a FRAM which attracts an attention as a non-volatile memory of next generation. Generally, the PZT is prepared by a gaseous phase film formation process such as sputtering, CVD, or by a liquid phase film formation process such as sol-gel method. For forming a multi-component thin film such as composite perovskite, the gaseous phase film formation process has a disadvantage in the aspect of securing homogeneity. On the other hand, the sol-gel method in which start material is of liquid phase has an advantage of securing a high homogeneity in the formation of the perovskite thin film.
To exhibit a performance as a FRAM, it is necessary for any PZT thin film formed by sol-gel method to have a film thickness of about 2,500 .ANG.. However, in any coating solution available at present, a film having a film thickness of only about 400 .ANG. to 500 .ANG. can be achieved by one coating operation. Therefore, in the prior art, when trying to obtain a PZT film having a film thickness of about 2,500 .ANG., it was necessary to carry out a multi-layer coating in which coating operation is repeated five to six times. This multi-layer coating has disadvantages of complicating the production process and giving a thermal damage to peripheral materials.